Cathode-ray tube



Jan. 22, 1952 F. H. TowNsr-:ND 2,583,500

cATHoDE-RAY TUBE Filed Nov. 25, 1949 Invehtor FREDERICK HENRY'FEWNSEND Attorney Patented Jan. 22, 1952 CATHODE-RAY TUBE Frederick II. Townsend, Cambridge, England, assignor to Cathodeon Limited, Cambridge, England, a British company Application November 25, 1949, Serial No. 129,232

In Great Britain November g5, 1948 3 Claims. (Cl. 313-82) The present invention relates to cathode ray tubes having magnetic deflection, such as are usedf in television apparatus.

In the manufacture of cathode ray tubes, considerable difficulty is experienced in mounting the lectron gun in accurate alignment with the axis of the tube so that the undeflected electron beam will be emitted along a path which is co-axial with the neck portion of the tube and will be equally spaced from the magnetic deflection coils positioned around that neck portion.l Due to inevitable slight inaccuracies in manufacture, the undeflected path of the emitted electrons is generally not co-axial with the neck portion, but is inclined to the axis.

Therefore, the focussing coil surrounding the neck y to follow a path which is parallel to the axis of the tube. However, since thereby the redirected beam is displaced to one side of the tube axis and thus lies nearer to the deflection coil on one side of the tube neck than to the deflection coil on the other side, the raster produced by the beam is distorted.

The present invention has for its object to provide a construction of cathode ray tube and method which overcomes these difficulties and enables cathode ray tubes to be manufactured on a mass production scale without the imposition of very close tolerances with regard tc the construction and location of the electron gun.

In order to avoid the raster distortion referred to, itis essential that the electron beam enter the field of the deflection coil yoke centrally therewith. To this end, according to the invention, a diaphragm is mounted within the neck of the cathode ray tube, in the plane at which the beam enters the deflection eld, that is after the beam has passed through the focussing field but before it passes through the deflection field. The diaphragm has an aperture which is carefully located on the axis of the tube neck.

'Ihe electron gun is so designed that the beam of electronsproduced is divergent, preferably widely divergent. The gun is then so located that the axis of its emergent beam is as nearly coincident with the axis of the tube neck as is reasonable by normal production methods and is directed towards the aforementioned diaphragm.

vBefore reaching the diaphragm, the beam passes through the focussing field and, owing to the effects of that field, the beam becomes convergent. As, however, the diaphragm is located only a short distance beyond the exit of this focussing field, thefbeam still has a diameter which is large relative to the diameter of the aperture. This diaphragm therefore allows only that part of the? electron beam to pass which is coincident with the aperture, and as a result of the careful location of this aperture the electrons emerging therefrom, which together form the operating electron beam of the tube, enter the dedection eld on the axis of the tube neck. It remains only to locate the deection coil yoke externally to the tube neck and centrally aligned with respect thereto, and the required result is achieved of obtaining a beam of electrons which enters the deflection field on the axis thereof whereby the raster distortions hitherto experienced may be avoided.

By this means, provided that the misalignment ofthe electron beam prior to its incidence upon the diaphragm is not so great that the portion of the beam allowed to pass through the aperture is selected from the edge of the beam where the electron. density is lower and less uniform than it is in the paraxial portion of the beam, the effects of such misalignment, be it due to mechanical misalignment of the electron gun or to the presence of asymmetrical fields having a deflecting action on the beam, are largely negatived.

In order that the invention may be more clearly understood, an embodiment thereof is diametrically shown in the accompanying drawing which illustrates an application of the invention to an otherwise f conventional type of cathode ray tube for television reception, having an envelope l comprising coaxial neck and bulb portions 2 and 3 respectively, the neck 2 containing an electron gun 4 of conventional construction comprising a cathode 5, a modulating grid 6 and an anode l mountedin the end of the neck andA directed towards a fluorescent screen 8 provided on the end face of the bulb 3 and upon which the raster produced by the beam generated by the gun is traced. A conventional focussing coil 9 and deflection coil yoke l0 are tted around the neck 2, and each aligned coaxially thereto. Il indicates the customary internal conductive wall coating of the bulb connected to anode potential.

The gun 4 is'located in the neck 2 so that the axis of its emergent beam, indicated at I2. is as nearly coincident with the axis of the neck as is reasonable by normal methods ofv production of the tube. The gun is designed so as cussing field and before it enters the deflec-Y tion eld.

In the operation of the tube, the divergent beam I2 emitted from the gun 4 passes Ithrough,

the focussing iield of the coil 9 and is made convergeht, as indicated at I5,- by ,the e'ects-Ofthatfield. As the diaphragm I3is located only a; short distance beyond the exit of the focussing eld, the convergent beam I 5 sti1-l-has-.a large. diameter relatively to the diameter f the'aper'ture I4, -"ai1'd consequently the diaphragm I3 allows only that part of the beam I which is coincident with' the aperture I4 to pass therethrough. By reason of the careful locationofthis aperture onthe neck axis, the emergent beam I5 enters thedeflection eld ofthe yoke'lll on theaxi's 'ofthe neck', Yand thus on the axisof the yoke lili and/ deflection ield, as a limited narrow angle"convergentfelec tron beam I6 which thus'tra'ces anundistorted raster on the screen 8.

Although the' invention has been illustrated in an application to a television-#receiving cathode ray tube of otherwise conventional construction', tho' described 'embodiment'is merely-given as an example, since the invention'is equally 'applicable to any type of cathode ray'tube in which the gern eration of a distortionlfree deflection pattern of a scanning electron beam is necessary;

Iclaim: m 1. A cathode'ra'y'tube having' an envelope in'- cluding a neck, an electron'gun including a cathode and electrode meansfor forming a'divergent beam of electrons'iroi'n "sad' cathode, said gun being mounted and aligned in said vneck to direct 'said beam substantially coaxially along said neck, a' focussing coil co'aXially-'mounted 'on said neck for focussing said divergent'beam into a con'- vergent beam, a diaphragm structurallyv independent Aof said gun mounted inand located with reference to said neck a short-distance beyond said coil for intercepting said coiciverg-ent beam emerging from the focussing eld ofsaidfcoil, said diaphragm having a small aperturev therein accurately located on saidx'neckv'axis to select from said convergentfbeamfandallowto pass through said aperture a paraxial portion of--said beamv coincidentwithsaid aperture, whereby -the selected beam emergent from s aid apertureof said diaphragm followsa pathcoincident Ywith said cluding a neck, an electron. gun including agcathode and electrode means for forming a divergent beam of electrons from said cathode, said gun being mounted and aligned. in, sadneck toj direct said beam substantially coaxially along said neck, a focussing coi1 coaxially mounted on said neck for focussing said divergent beam into a convergent beam, a plane diaphragm structurally independent of said gun mounted in and located with reference to said neck perpendicularly to said neck `axis a-short.-distance beyondgsaid coil for intercepting said convergent beamemerging from the focussing field of said coil, said diaphragm having a small circular aperture therein accurately located on said neck axis to select from said convergent beam and allow to pass through said aperture a paraxial portion of said beam coincident with said aperture, whereby the selected beam emergent from said aperture of said diaphragm follows a path coincident with -said'neck axis, and deflecting coil means mounted coaxially on said neck beyond said diaphragm for d'eilecting said emergent beam to form a raster.

3. A cathode ray tube having an envelope includinga neck, an electron-gun including a cathode and'jelectrode means for Aforminga divergent beam of electrons from said cathode, said gun beingmounted and aligned in said neck to direct saidbearn substantially coaxially along said neck, a focussing coil coaxially mounted on said :neck for' focussing said divergent beam into a convergent beam, a diaphragm structurally independent of said gun mounted in and located with reference to' said neck for ntercepting said convergent beam emerging from the focussing .field offsaid coil at a'short distance beyond the Zone of inflection of said beam, said diaphragm having a'fsmall aperture therein accurately located :on said neck axis to select froml saidconvergent beam and allow to pass through said aperture a paraxial vportion'ofysaid beam'. coincident with `said aperture, whereby the selected beamy emergent from said aperture ofA said diaphragm follows apath 'coincident with said 4frreck axis-,tand deflecting coil means mounted coaxial'ly onsaid neck for deflecting said' emergent' beam to form a raster, said' deecting means being located such -th'atsaid diaphragm lies in the 'plane at which said emergent beam enters the deflection iield of said delecting means. f 1

' FREDERICK H. TOWN SEND.

REFERENCES CITED i The Afollowing references are of record in vthe iile of this patenti UNITED STATES P .ATnN'rs 

